1. Technical Field
The present invention relates to an improved tunnelled cone mirror to be installed in a sonde moving up and down in a borehole bored into ground for observing at a time the whole circumference of the wall of the borehole at an arbitrary depth in order to inspect the surface thereof, a compound type cone mirror, a combined mirror for locally observing the hole wall in detail, and an elevating sonde equipped with these mirrors.
2. Background Art
In a geological survey, it is required to know the state of directional continuity of plane elements such as of bedding, joints and cracks. For this reason, during said geological survey, a sonde has hitherto been moved up and down in the borehole, and a borehole television camera installed in said sonde has been used for directly observing the hole wall. However, with said sonde with built-in borehole camera photographing an image of the hole wall reflected at a plane mirror slanted at an angle of 45.degree. to the surface of the hole wall, the range which can be photographed at a time is limited, which makes it difficult to obtain an overall picture of the underground structure in the borehole.
A device as shown in FIG. 23 was developed, wherein the whole circumference of a part of the side wall of a cylindrical elevating sonde (30) is formed as a transparent window (31), a tunnelled cone mirror (131) which has a tunnel-like hole (130) piercing through in the actual direction at the center of a truncated cone block the conical side face of which is mirror finished by plating etc., is mounted inside of the window such that the incident light incoming through said transparent window (31) from the whole circumference of the borehole wall is reflected into the axial direction, a needle azimuth meter (34) is provided on the axis below said cone mirror (131), and further a television camera (36) for taking a photograph of said azimuth meter (34) through said reflected light and said tunnel-like hole (130) is installed above said cone mirror (131).
With this device, however, particularly the maintenance of the cone mirror was difficult. For this reason, an improved type as shown in FIG. 24 is in use, wherein a lower cone (32) which is designed to be readily attached to and detached from the sonde is provided at the tip of the sonde and constitutes a support for said tunnelled cone mirror (131), so that taking-off the cone mirror from the sonde is facilitated. However, both of said sondes have problems as follows:
(1) The conical mirror face of the cone mirror is exposed to the outside and is thus liable to be contaminated and to become scratched.
(2) The television camera and the mirror are separated, and thus aligning both of them in axis, focus, etc. is time-consuming.
(3) The cone mirror is situated at a position where it is liable to be cooled, and thus dewdrops are likely to be formed on its conical mirror face. Further, both of said devices allow to observe the whole circumference of the hole wall rapidly and easily, but on the other hand they are not suitable when it is desired to observe the detailed structure of a part of the wall surface.